The onset of longitudinal wrinkles is determined by the out-of-plane displacement along the midline of the ribbon, as illustrated in Fig. REF a. After the onset of wrinkling, we observe that the frequency content of the wrinkle profile is peaked at a characteristic frequency \(f=1/\lambda \) , with \(\lambda \)  being the dominant wrinkle wavelength (Fig. REF b). To enhance the detection of wrinkle onset, frequencies outside a specified range near the characteristic frequency are removed from the wrinkle profile, as shown in Fig. REF b–c. The resulting wrinkle amplitude exhibits a sharp increase at a scaled twist angle \(\eta _{\text{lon}}\) ; this “knee point” [1]} of the amplitude curve is taken to be the onset of longitudinal buckling (Fig. REF d).
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